Every day, cargo is shipped around the country in environment-controlled transport units such as trucks, semi-trailers, railroad cars and other cargo containers. The environment-controlled transport units regulate the environmental conditions, such as the temperature and humidity, of the container's enclosed space, or “conditioned space.” Environment-controlled transport units allow many kinds of products to be transported over great distances without spoilage or damage. An environment-adjusting system, such as a refrigeration system, is used to set and regulate the environment of the conditioned space. A control unit governs the environment-adjusting system.
An early control unit was a simple thermostat, which turned a refrigeration system on and off to keep the temperature of a conditioned space close to a desired temperature. A simple thermostat employed only one environment-control parameter, i.e., the desired temperature of the conditioned space, or “set point temperature.” An operator simply set the thermostat to the desired set point temperature, and the thermostat controlled the refrigeration system to create and maintain the set point temperature in the conditioned space.